2 DIFFERENT GENES ENCODE FERROCHELATASE IN ARABIDOPSIS - MAPPING, EXPRESSION AND SUBCELLULAR TARGETING OF THE PRECURSOR PROTEINS

Ferrochelatase is the last enzyme of haem biosynthesis. We have isolat ed 27 independent ferrochelatase cDNAs from Arabidopsis thaliana by fu nctional complementation of a yeast mutant. Twenty-two of these cDNAs were similar to a previously isolated clone, AF3, and although they va ried in length at the 5' and 3' ends, their nucleotide sequences were identical, indicating that they were derived from the same gene (ferro chelatase-I). The remaining five cDNAs all encoded a separate ferroche latase isoform (ferrochelatase-II), which was 69% identical at the ami no acid level to ferrochelatase-I. Using RFLP analysis in recombinant inbred lines, the ferrochelatase-I gene was mapped to chromosome V and that for ferrochelatase-II to chromosome II. Northern analysis showed that both ferrochelatase genes are expressed in leaves, stems and flo wers, and expression in the leaves is higher in the light than in the dark. However, in roots only ferrochelatase-I transcripts were detecte d. High levels of sucrose stimulated expression of ferrochelatase-I, b ut had no effect, or repressed slightly, the expression of the ferroch elatase-II isoform. Import experiments into isolated chloroplasts and mitochondria showed that the ferrochelatase-II gene encodes a precurso r which is imported solely into the chloroplast, in contrast to ferroc helatase-I which is targeted to both organelles. The significance of t hese results for haem biosynthesis and the production of haemoproteins , both within the plant cell and in different plant tissues, is discus sed.